Optical phased array (OPA) device are known in the prior art, but they have grating lobes in their output beams.
The presence of grating lobes is one of the main issues with prior art optical phased arrays due to the difficulty of fabricating arrays with spacings less than wavelength. This problem is overcome by the present invention. The present invention provides additional benefits, including a narrow beamwidth of the output beam together with high beam position accuracy.
The prior art includes a liquid crystal-based optical phased array (LC-OPA) which consists of a liquid crystal (LC) cell with one-dimensional patterned transparent conductor strips in which each strip defines an element of the linear array. For beam steering in two dimensions, two such LC cells are arranged in orthogonal orientations. LC-OPA is a fairly mature technology with very low power consumption due to the capacitive nature of the liquid crystal. However, the main issue with the LC-OPA is its slow steering speed (10's of ms range) which is due to the slow response time of the LC-based phase shifting elements. Another disadvantage of liquid crystals is their limited temperature range of operation. At low temperatures (<20° C.) the LC response time significantly degrades due to its increased viscosity, while at higher temperatures (>50° C.) it becomes isotropic and hence loses its functionality. See P. F. McManamon et. al., Proceedings of IEEE, Vol. 40, No. 2 (1996) p. 268.
Another prior art optical beam steering approach is based on the use of integrated semiconductor waveguide arrays in which each array element is a tunable phase shifter. The phase tuning is achieved via the linear electro-optic effect in the material. The main problem with this approach is that it is mainly a 1-D phased array. 2-D beam steering can be achieved by placing tunable waveguide gratings at the end of the phase shifters, but scanning in the elevation direction would be very limited (<1°). In principle, one can envision an in-plane 2-D array of optical phase shifters coupled to an out-coupling grating structure for 2-D beam steering. However, this would become a very complicated structure to realize since it is difficult to route N2 in-plane phase shifted beams to a 2-D array of N2 vertically emitting elements on the same substrate. See F. Vasey et. al., Applied Optics, Vol. 32 (1993) p. 3220.
The approach which is arguably closest to the V-OPA device disclosed here that has been investigated is based on self-locking of a VCSEL array using evanescent coupling between adjacent VCSELs, and varying the injection current of individual VCSELs for phase shifting. See, A. C. Lehman et. al., Applied Physics Letters, Vol. 88 (2006) p. 021102. The problem with this approach is that strong phase locking occurs only among nearest neighbors. This can lead to mode hopping and instabilities for VCSEL arrays with large numbers (>10) of elements. Varying the drive current of each VCSEL with respect to others in the array can result in some phase shifting. However, the level of phase tuning of evanescently coupled VCSELs with drive current has been shown to be less than π/2, insufficient for a high performance phased array. Furthermore, current tuning of VCSEL phase results in its amplitude variation as well.